1. Field of the Invention
This invention relates to a copper oxide superconductor having novel particle characteristics and a process for its production, and also to a novel copper complex and a process for its production.
2. Description of the Prior Art
Generally, Bi-based superconductors are used to produce wire and tape materials of copper oxide superconductors. For example, in the wire, tape and bulk section of Fourth International Superconductor Symposium, there were three reports using a Y-based superconductor, whereas there were 13 reports using a Bi-based superconductor. This was because crystal particles of Bi-based superconductors have a flat particle shape and are easily particle-oriented by a machine processing and a large critical current density is obtained since the flowing direction of a superconducting current becomes uniform. Actually, by this technique, a critical current density of at least 10.sup.4 (A/cm.sup.2) is obtained at 77K (see WBP-27, 30, 39).
When a Y-based copper oxide superconductor is used for wires or tapes of a high temperature superconductor, it is known that its particles are bulky and are not oriented by a mechanical processing, and therefore, its critical current density is small. The above three reports are on 123-type superconductors, a kind of a Y-based copper oxide superconductor. Two of them form a wire material directly from a gel condition without a mechanical processing after formation of a superconductor (WBP-21) or form a superconductor tape material directly on a supporting carrier tape by a method of thin film formation (WBP-36). The remaining one report comprises forming a superconductor, adding a silver powder, and pressing the mixture by a roll (WBP-22). It only describes a critical current density of 2.1.times.10.sup.4 (A/cm.sup.2) (at 77K) calculated from magnetization characteristics, and does not describe a value measured by actually flowing a current. The critical current density calculated from the magnetization value reflects the critical current density in one individual particle. It is especially known that in a sample which has gone through a partial melting process, it is considerably larger than the value measured by actually flowing an electric current (the critical current density of a sample as a whole containing a weak bond). It is therefore expected that the critical current density of the sample pressed by a roll is considerably lower than the above-mentioned value. In the case of a Y-based copper oxide superconductor, its particles are generally not flattened and are not oriented by a technique of making wires and tapes by mechanical processing. Accordingly, superconductor wires, tapes and thin plates having a high critical current density cannot be produced. On Y-based copper oxide superconductor 123-type superconductors, there is only one report that plate-like crystals were produced Ozaki et al., Nikkei Superconductors, Jul. 25, 1988, p.9). But its zero resistance temperature Tc(R=9) is about 80K, and its application at a liquid nitrogen temperature (77K) is not satisfactory.
Heretofore, the synthesis of copper oxide superconductors is performed in the prior art mainly by a solid phase reaction method, a liquid phase reaction method (sol gel method, coprecipitation method), and a gaseous phase reaction method (sputtering method, evaporation method, CVD method). The solid phase method is simple in process and easy to synthesize. The liquid phase reaction method has a possibility of synthesizing at low temperatures, and of imparting shapes such as bulk, films and fibers. Furthermore, the gaseous reaction method can form a thin film. Thus, the three synthesizing methods have respective characteristics, and can be used according to the purposes.
However, in the liquid phase reaction method, synthesis at low temperatures or imparting of shapes have not been performed sufficiently. For example, in syntheses at low temperatures, the synthesis of a 124-type superconductor (YBa.sub.2 Cu.sub.4 O.sub.8) as one of the copper oxide superconductor has been cited. It is reported that this 124-type superconductor (YBa.sub.2 Cu.sub.4 O.sub.8) (Nature, vol. 336 (1988), p. 660) is a high temperature superconductor having a class of 80K, and by substituting a part of Y by Ca, Tc rises to 90K (Nature, vol. 341 (1989), p. 660). The 124-type superconductor containing Ca is synthesized under a high oxygen pressure, and has a relatively high Tc and is thermally stable. It is considered to be a practically important material. But its synthesis requires treatment under a high oxygen pressure, and has difficulty. Recently, it has been reported that a 124-type superconductor containing Ca was synthesized at a relatively low temperature (820.degree. C.) under atmospheric pressure (Physica C, vol. 173 (1991), p208). However, its Tc was 85K, and an increase of Tc by Ca doping is small, and the transition of the superconductivity is broad. This is because a temperature of 820.degree. C. is considered to be too high for the synthesis of a 124-type superconductor.
As to the imparting of shapes, precursors such as bulks, films and fibers of a copper oxide superconductor have not been produced by hydrolysis and polymerization of an alkoxide material.
A sol gel method using an alkoxide is an excellent technique of being able to synthesize at low temperatures or to impart shapes by synthesizing uniform precursors at an atomic level by hydrolysis and polymerization. However, in the prior synthesis of copper oxide superconductors, suitable alkoxide materials for copper as an essential element are not available, and suitable hydrolysis and polymerization cannot be performed, and it has been impossible to perform synthesis at low temperatures or impart shapes. For example, when the Murakami's method of synthesizing a Ca-free 124-type superconductor at low temperatures under normal pressure (Japan J. Appl. Phys., vol. 29 (1990), p.2720) is applied to the synthesis of a Ca-containing 124-type superconductor, an alkoxide material is used for elements other than Cu, and a nitrate salt material is used for Cu. The water of Cu nitrate hydrolyzes other alkoxide material instantaneously and precipitate it as a powder. The nitrate group selectively binds to Ba and precipitates. As soon as the nitrate salt material of Cu is mixed, all metal components become powdery, and cannot be made into fibers. The inventors have found that the effect of Ca doping cannot be sufficiently realized (Tcon=85K). We have also found that general alkoxide materials for Cu, such as a methoxide, propoxide, and butoxide, do not have a solubility of a practical level.